PROJECT SUMMARY/ABSTRACT This application is a competitive renewal of a longitudinal study of infants at risk for autism (R01 MH068398, 7/1/03 - 6/30/08). In the first five years of funding, a cross-site study (UC Davis and UCLA) was conducted in which 180 siblings of children with autism were compared to 90 siblings of children with typical development on a wide array of behavioral tasks measuring constructs important to the very early autism phenotype. In this competitive renewal, we request additional funds to recruit a new sample of infant siblings and to follow longitudinally the original sample as they enter school. Based on our previous findings of few differences in behavioral symptoms at 6 and 12 months of age, experimental measures used with the new sample will focus on underlying processes that may reveal differences in at-risk children prior to the onset of behavioral signs of disorder. Collaboration with UCLA will continue for both the new sample and the longitudinal follow-up of the previous sample. Specific aims are to: AIM 1: Identify reliable markers that differentiate children with autism or ASD outcomes from children with other or no developmental delays, examining both onset of symptoms and potential underlying mechanisms to see when trajectories diverge and in what domains (Study 1: newly ascertained infant sibling sample). AIM 2: In the sample of siblings of children with autism recruited in the prior award period, examine stability of autism and PDDNOS diagnoses and identify later-appearing developmental problems by assessing functioning at age 6 (Study 2: longitudinal follow up of previous infant sibling sample).